1. Field of the Invention
This invention relates to a milling tool combined with a stabiliser therefore, the milling tool being adapted for milling tubular members especially down hole.
2. Description of the Related Art
It is known to provide a milling tool in a drilling string with a stabiliser but the stabiliser tends, in such a configuration, to be too far displaced from the milling tool.
A known combined milling tool and stabiliser is shown in FIGS. 1 and 2 in which FIG. 1 is a side view of a casing milling tool combined with a stabiliser and FIG. 2 is a cross-section along double arrow headed lines A--A.
In the FIGS. 1 and 2 a casing milling tool has a tubular body 1 having a bore 2 therethrough for circulation of mud and six equi-circumferentially radial blades 3 located about the exterior surface of the body 1. Each of the blades 3 is secured, for example by welding, to the body and each of the blades has a leading face 4, that is, facing forwardly in the intended direction of rotation, and a radially outer edge 5 which are coated with crushed tungsten carbide particles randomly dispersed in a bed of brazing material. Crushed tungsten carbide 6 is also provided on a trailing, radially outer, corner 6 of each blade. The outer edge 5 typically has a 5.degree. angle 7 and a 10.degree. attack angle 8. Interspaced between alternate blades 3, are optionally provided stabilising fins 9 each of which has a radially outer edge coated with crushed tungsten carbide.
The pilot stabiliser section 10 is formed by four equi-circumferentially spaced fins 11 each of which are helically formed and located about approximately 90.degree. of the outer circumference of the body 1. Each of the fins 11 is secured, for example, by welding to the body 1 and each of the radially outer edges 12 of the fins 11 is coated with crushed tungsten carbide 15.
It will be seen from FIG. 1 that the pilot section 10 is integrally formed with the section formed by the casing milling tool and both milling tool and pilot stabiliser section 10 are integrally secured to the body 1. A lower, in use, portion 13 of the body 1 has an internal screw thread thereon for securement to other drilling string members and the upper, in use, end of the body 1 has an internal screw thread 14 for securing the body 1 to a drilling string.
Referring now to FIG. 3 which shows a vertical cross-section through a tubular member pipe casing, an outer casing 20 supports an inner casing 21 which is to be milled with there being a casing coupling 22 at an upper end of the casing 21. The space between the casings 20 and 21 is normally filled with cement 23 and, as often occurs, the casing 21 having a longitudinal axis 24 is offset from the longitudinal axis 25 of the casing 20. In FIG. 3 it will be seen that the mill blades 3 are abutting the inside circumference of the casing 20 on the right hand side (as shown in FIG. 3) and the pilot stabiliser 10 is abutting the left hand side of the inner circumference of the casing 21 and the longitudinal axis 26 of the body 1 is offset from both axes 24 and 25.
Thus, it will be appreciated that the radial height of the fins of the pilot stabiliser will need to be varied in dependence upon the offset of casing 21 within casing 22. Hitherto, it has been necessary to either take a number of milling tools each, as shown in FIGS. 1 and 2 and each with different pilot stabiliser fin radial heights onto a drilling site, or, more usually to take a single milling tool, as shown in FIGS. 1 and 2 and to oxyacetylene cut the fins to the required height when on site. It was then necessary to redress and grind the pilot stabiliser and it very often occurred that the fins would crack due to frequent welding so that the life of the tool was greatly reduced.
The present invention seeks to overcame the foregoing disadvantages whereby time on site is substantially reduced and the need to have a large inventory of casing mills with various outside diameter stabilisations is avoided. It is accordingly an object of this invention to provide an improved combination milling tool and stabiliser therefor.